The use of circulating fluidized bed systems for burning carbonaceous materials to generate steam from heat exchangers disposed within fluidizing reactors is well documented throughout the literature.
Circulating fluidized bed systems involve a two phase gas-solids process which promotes solids entrainment within the upflowing gas stream in the reactor chamber and then recycles the solids back into the reactor chamber with a high rate of solids circulation. The rate of solids circulation in the circulating fluidized bed process is about fifty times that of a bubbling bed process. Circulating fluidized bed systems differ from bubbling bed systems in that they typically employ air to move the fuel in a circulating path as it combusts which increases turbulence, residence time, and a higher bed temperature in contact with the heat absorbing surfaces throughout the combustor system, thus increasing carbon combustion efficiency, increasing heat transfer throughout the system and decreasing carbon monoxide emission levels.
Various examples of known circulating fluidized bed systems are described in U.S. Pat. Nos. 4,165,717 (Reh et al.) and 3,625,164 (Spector), and an article by A.M. Leon and D.E. McCoy, presented at the First International Conference on Circulating Fluidized Beds, Halifax, Nova Scotia, Canada (Nov. 18-20, 1985), and entitled "Archer Daniels Midland Conversion to Coal."
Common to all circulating fluidized bed systems is the need for a fluidized bed reaction chamber or combustor, a solids separation unit and a solids re-injection unit. All of the circulating fluidized bed systems disclosed in the above patents and article have particle separators (i.e., cyclones) and solids re-injection units (i.e., FluoSeals.RTM.) which are disposed external of the reaction chamber.
In most conventional systems the combustor, particle separator and solids re-injection unit are supported independently. Thus, requiring their own structural support systems, refractory protected flue gas ducts with expansion joints and in some designs separate water-cooled or steam-cooled systems with their own supply and relief piping.
Still other have attempted to provide a combustor reactor having a particle separator disposed within the reactor chamber. Some examples of these integral combustor/separator systems are shown in U.S. Pat. No. 4,546,709 (Astrom), which issued on Oct. 15, 1985, U.S. Pat. No. 4,301,771 (Jukkola et al.), which issued on Nov. 24, 1981, U.S. Pat. No. 5,117,770 (Hassinen), which issued on Jun. 2, 1992, and U.S. Pat. No. 5,070,822 (Kinni et al.), which issued on Dec. 10, 1991.
The Astrom patent discloses a reaction chamber with a fluidized bed and a cyclone separator disposed within the reaction chamber in order to attain a high degree of separation, which is desirable if the gases are to pass through a turbine.
The Jukkola et al. patent discloses a fluidized heat exchanger which comprises a housing, a reaction chamber and a convection heat exchange chamber above the reaction chamber. The convection heat exchange chamber is separated from the reaction chamber by a slanted baffle. The baffle defines a gas passageway between the reaction chamber and the convection heat exchange chamber. At the bottom of the baffle is a hopper portion wherein dust is collected and removed from gases passing through the convection heat exchange chamber.
The Kinni et al. and Hassinen patents each disclose a reactor chamber having a particle separator disposed therein. The particle separator has an outer casing and an inner casing. These casings both have a circular horizontal cross-section and center lines which are arranged essentially to coincide with the center line of the reactor chamber.
The Astrom, Kinni et al. and Hassinen patents each require that an independent particle separator unit be built and installed within the reaction chamber. This is extremely costly, technically difficult to install and extremely difficult to service.
The baffle design of Jukkola et al. relies solely upon gravity for particle separation of large particles, but does not provide sufficient means for separating fine particles from the flue gases. That is, the combustion gases exiting the combustion chamber have entrained therein a substantial amount of dust. In traversing the convection chamber the larger particles of unburned fuel, ash and limestone drop out and are collected by the hopper. These particles slide down the inclined surfaces of the baffle and hopper into the ash conduit for recycling into the fluidized bed. The gases leaving the reactor vessel through the exhaust gas conduit will have entrained therein a substantially reduced amount of fine dust but it may be desirable to provide a cyclone external to the reaction vessel to capture these fine solids and return them to the fluidized bed through a cyclone return conduit. Therefore, the slanted baffle of Jukkola et al. does not remove sufficient amounts of fine particles to avoid the need altogether of an externally disposed cyclone or particle separator. Moreover, the fluidized bed boiler of Jukkola et al. is directed primarily to a bubbling bed system rather than to a circulating fluidized bed system and is not concerned with the recycling of a substantial portion of entrained solid particles since it does not generate nearly as much entrained solid particles as normally associated with a circulating fluidized bed system.
The present invention is directed to a circulating fluidized bed boiler system which is a single integrated or integral fluidized bed reactor, solids separation unit and solids re-injection unit. These three normally separate and independent components are combined into a single integral system within the water-cooled combustor system. The present inventor has discovered that incorporating these components into a single integral system simplifies the structural support and eliminates the high maintenance refractory lined breeching with expansion joints.
The integral circulating fluidized bed boiler system according to the present invention also provides the following advantages over conventional multiple component systems: (1) lessens the complicated structural supports for the various units, (2) eliminates expansion and sealing problems between the reactor and particle separator and between the solids re-injection unit and the reactor, (3) reduces the cost of manufacturing and field construction, (4) reduces the required plan area thus saving real estate costs, (5) provides a larger range of boiler applications, (6) reduces maintenance costs and requires that fewer replacement parts be stocked, and (7) simplifies the exterior maintenance platforms and walkways.
Also, the unique arrangement of the system of the present invention allows the conventional back-pass convection superheater/economizer unit to be incorporated into the water-cooled membrane enclosure thus providing one uniformly expanding singularly supported piece of equipment.
The present invention also provides many additional advantages which shall become apparent as described below.